Low voltage wire mesh heating element

ABSTRACT

A method of heating a surface in which a wire mesh or netting heating element is spread evenly over or just under an area to be heated, and through which an electrical current is driven. The two ends of the heating element are connected to the secondary terminals of a transformer, which converts the supply voltage to a low and safe level.

TECHNICAL FIELD

This invention relates to the heating of large surfaces.

BACKGROUND ART

European Patent 0048608 describes a heating concept wherein low voltageelectricity is passed through bare uninsulated electrical conductorsinto a medium which contains a large volume of non-gaseous material forstoring the heat for gradual dissipation. The apparatus described inEuropean Application No. 0048608 by Charlton Thermosystems is primarilydesigned to store and dissipate heat.

The Charlton Thermosystems concept is to heat a medium with an embeddedlooped heating element. The heating elements are in the form of steelbars, wire rope, and, it is said, can be embedded in a medium or a basematerial for the medium. Heat is not dissipated uniformly over a givenarea.

An object of the present invention is to use low voltage electricity toinstantaneously heat large surface areas using similar transformationtechnology to that described in European Patent No. 0049608.

Further objects and advantages of the present invention will becomeapparent from the ensuing description.

According to the present invention there is provided a method of heatinga surface by means of wire mesh or netting heating element spread evenlyover or just under the area to be heated through which an electricalcurrent is driven comprising connecting the two ends of the element tothe secondary terminals of a transformer which converts the supplyvoltage to less than 40 volts.

The mesh heating element is elongated and between 10 m and 100 m long,and has a width of between 75 mm and 1200 mm.

Preferably the wire mesh or netting thickness is less than 2 mm.

The heating element can be uninsulated and encased in a settable medium.

The heating element can provide 2500 cm² to 6000 cm² of surface per 100watt output for every square meter to be heated.

According to a further aspect of the present invention there is provideda method of heating a surface by means of wire mesh or netting heatingelement spread evenly over or just under the area to be heated throughwhich an electrical current is driven comprising connecting the two endsof the element to the secondary terminals of a transformer whichconverts a supply voltage of the order of standard mains voltage voltsto less than 40 volts at 16 amps to 240 amps current throughputdepending on size.

The mesh heating element may be elongated and may be between 10 m and100 m long, and have a width of between 75 mm and 1200 mm, depending onthe size and dimensions of the area to be heated. Wire thickness may beless than 2 mm diameter which requires only a very thin layer of grout,bedding compound, adhesive or plaster where particular floor surfacesrequire such a backing.

The voltage across the heating element is preferably less than 40 voltsand in most cases will be around 30 volts so the dimension of the coppercable is compatible with the wire in the mesh.

Steel is used as both conductor and heating element and is commonlyuninsulated but may be encased in grout, bedding compound, adhesive,plaster or PVC sheet etc to hold the elements in position.

Cross wires may be used to:

1. Conduct heat from hot spots.

2. Conduct electricity around the hot spots.

3. Hold the wires in a uniform layout.

4. Double the heat dissipating surface area of the conductor.

The method can provide a heating element with some 2500 cm² to 6000 cm²of surface per 100 watt output for every square meter to be heated.

The heated surface may be a floor, wall and ceiling of a building, yardor vessel.

The special value of the mesh heating is the very even spread of heatover large surface areas, the great strength of the heating elementitself which provides security of the system against rough handlingduring installation, surface medium cracking or movement during use, andthe low voltage electricity used, providing electrical safety againstcontact with the heating element and/or soaking in water.

The invention is specifically aimed for use in providing 24 hour per dayswitch on/switch off heat to carpet, vinyl, linoleum, tiled and grout orlevelling type compound surfacing. The system could be used for manyother surface heating requirements and may be used for supplying heat toa surface for only short periods of time if that is what is required.

The mesh proposed to be used is galvanised welded wire mesh constructedof 18 Birmingham wire gauge wire on a square grid 25.4 mm, 19 mm or 12.7mm centres and the mesh is presently supplied in rolls 915 mm wide by30.5 m long. However mesh of other dimensions could be used provided thetransformer was designed to match the heating element dimensions.(Diamond woven mesh--`chicken netting` can also be used and may evenperform better in some situations).

A typical heating element made from this mesh will have a surface areaof up to 5760 cm² per sq meter in contact with solid medium in which itis embedded. Steel rope or wire method is unsuitable for use right atthe surface because the floor would have warm lines at say 30 to 50 cmcentres with cold patches between and could damage the surface by meansof the uneven heating and cooling. On the other hand the presentinvention spreads the heat evenly and has the transverse wires toconduct heat laterally and reduce hot spots that might be caused by somematerial of poor conductivity laying on the heated surface.

By using cross wires in the circuit the heat dissipation is speeded upand the wires will not normally reach more than 40 degrees C. Localoverheating along the circuit due to a faulty wire or join wouldnormally produce an increase in resistance and more heat coming into thespot which is already hot. In the system of this invention, the crosswires will conduct electricity around the higher resistance hot patch,putting less power into the spot as well as conducting heat from thearea via the wires. This is an important safety feature since theheating system is expected to be used in living rooms and bedrooms whereblankets, clothes, rugs etc could overlay the heating surface and is notoffered by the technology described in European Application No. 0048608.

Although in most circumstances it is advantageous to have the mesh bareand uninsulated, there are some situations where there could be arequirement to have it insulated such as when the surface to be heatedis to be a good conductor of electricity. In these cases the mesh couldbe plastic coated. In a corrosive environment where the galvanising isvulnerable, it may also be advantageous to have a plastic coating.

Because of the high electrical currents involved in the heating circuit,low resistance at the joining of the conductors is critical to thesafety of the equipment. Joints in the heating element for small areashave been minimised by a method of cutting sheets of mesh into acircuit.

Each end of the heating element is normally welded to a copper,aluminium or steel strip or cable of some 15 to 30% the resistivity ofthe heating element. These are called "cold leads" and each one is inturn bolted or crimped to the transformer terminals in a similar way tohow a motor vehicle's electrical starting circuit is connected to thebattery. The cold leads are normally insulated.

The transformer that is proposed to be used is any double woundisolating transformer capable of converting the input voltage to under30 V and safely handling the continuous power consumed.

The transformer primary circuit may be fitted with a thermocoupletemperature cut-out and an overload cut-out in case of short circuit.They will also normally have switches connected to various tappings sothat one of several heat outputs can be selected. In some situations theinput power may be controlled by a thermostat. It will be usual to havean indicator light to show when the heater is on. A rectifier may beused in situations where a direct current is an advantage.

This invention is directed specifically towards heating under carpets inexisting buildings and the following features are specific to that end.

The invention/design is such that existing carpet can be lifted and theheating element placed and fixed where required and the carpet re-laid.

The idea of low voltage heating through steel elements has been aroundfor many years but this invention provides the means of spreading theheat more uniformly and more efficiently than any other system.

To our knowledge no other low voltage system has been able to be useddirectly under the carpet.

1. The invention uses welded steel mesh to provide a flat heatingsurface of large area. The thinness of the element, less than 2 mm, andclose spaced wire grid 25.4 mm×25.4 mm or 19 mm×19 mm or 12.7 mm×12.7 mmensures that foot traffic will not detect the heating element and alsothat no filler or hard covering is required.

2. The invention provides a method of using standard width, wide sheetsto achieve the length of circuit with minimum joints.

3. A method for holding elements in position during transport andinstallation and preventing short circuit is provided wherepolypropylene or polythene sheet under the element is glued or fused tostrip, sheet or patch over the mesh each side of the space betweenelements and where a cut is made the wires are bent in such a mannerthat when taped the rows of wire stubs resist any tendency of the rowsto move towards each other.

4. Where joints are required the invention provides strong methods ofjoining with minimal brazing and which avoids fire risk withoutsignificantly increasing the thickness of the element.

5. The invention further provides a method of joining the copper coldleads to the mesh without risk of fire or significantly increasing thethickness of the elements by using stranded copper cable with strands ofsimilar dimension to the steel wire in the mesh or flat copper sheet forlarger units where the current is very high and which are welded to themesh in a manner which maintains the integrity of the circuit and thethinness of the heating element.

6. A circuit layout with a plurality of legs is provided, where only oneleg of the circuit has to be insulated from the rest to prevent seriousshort circuit. (Short circuits between two adjacent legs would not haveserious consequences).

BRIEF DESCRIPTION OF DRAWINGS

Aspects of the present invention will now be further described withreference to the accompanying drawings in which:

FIGS. 1 and 2: illustrates possible heating element layouts, and

FIG. 3: illustrates a further possible heating element layout, and

FIGS. 4 and 5: shows the use of a plurality of heating elements to heata particular area, and

FIG. 6 is an enlarged view of the circled area of the wire mesh labeled6 in FIG. 4, showing adjacent heating elements;

FIG. 7 is a further enlarged view of the circled area of FIG. 6,illustrating the cut areas in more detail;

FIG. 8 is an enlarged view of the circled area labeled 8 in FIG. 4,showing brazing of two adjacent sections of mesh

FIG. 9: shows how copper leads can be joined to heating elements, and

FIG. 10A illustrates formation of a diagonal cut in a heating element;

FIG. 10B illustrates welding of a joint between the cut sections of FIG.10A;

FIG. 11: is a plan view of a heating element for heating a small area,and

FIGS. 12 and 13: are side and plan views showing apparatus andmethodology for manufacture of the elements of FIG. 11, and

FIG. 14: is an end view of parts of the apparatus of FIGS. 12 and 13.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIGS. 1, 2 and 4, cold leads 1 (as described herein) on the outputside of transformer 2 are connected to spaced apart elements 3 all ofwhich are electrically connected and form a continuous circuit. As isimplied the mesh elements 3 may be in many different sizes and shapes.As best illustrated in FIGS. 6 and 8, the mesh heating element 3comprises a plurality of longitudinal wires 14 connected togetherthrough welding to a plurality of cross wires 15.

In FIG. 3 the elements 3 are laid out to heat exposed floor area in akitchen, laboratory or such like which includes furniture items such ascupboards, or benches 4.

FIG. 4 illustrates how a 1 kVA transformer can be used to heat a 3.2m×3.4 m area using welded wire mesh cut and welded to form a 200 mm wideheating element over 50 m long. In this case the output voltage is 30Vand the element is constructed from 18 B W G mesh on a one inch grid.The same element layout could be used for a 2 ka transformer with 30Voutput to heat an element constructed of 18 B W G mesh on a half inchgrid. In this case the heat output would be twice as much per squaremeter.

FIG. 5 illustrates a method for heating a very large area such as a hallor shop floor 16 with mesh heating elements 3.

FIG. 6 shows part of the circuit of FIG. 4 in more detail. Asillustrated in FIG. 6, cuts are made in the mesh to provide an elongatedcircuit of mesh.

FIG. 7 shows how the cut is formed and taped in a way that inhibitsmovement that would tend to make a short circuit. The cut wires are bentto form bent ends 20, as illustrated in FIG. 7, such that the cut endswill not touch even if the adjacent legs of the mesh close up slightly.A patch 21 of tape is adhered over the cut ends to further inhibitmovement which could otherwise result in a short circuit.

FIG. 8 shows how a standard joint is brazed. Cross wires or wire links26 are used to hold the conducting wires in a uniform layout. Weldpoints 28 join the wire links to the mesh 3 at each end.

FIG. 9 shows how the copper leads of the FIG. 4 embodiment may be joinedto the mesh and how the one leg of the circuit may be insulated toprevent a short circuit to other legs. As illustrated in FIG. 9, copperleads 1 are applied to the tops of two adjacent sections of mesh andwelded at weld points 30. Insulation 32 is included to provideelectrical isolation between the two banks or legs of mesh 3.

FIGS. 10A and 10B show a method of turning a length of element 90degrees without increasing the thickness of the element itself. Theelement is cut at 45 degrees along line 22 as indicated in FIG. 10A,halfway between the mesh welds. One section is turned over and the joinmeshed together so that one solder point 24 will join four wires asillustrated in FIG. 10B.

A variation on the method of heating is adopted for heating a very smallarea, 0.125 m² by means of using a standard car battery charger. As isknown in the field, a standard car battery charger can be more preciselydefined as a transformer capable of converting mains voltage to between6 and 24 volts and which has less than 150 VA capacity. The batterycharger output terminals are connected to a 10 m length of smalldiameter steel wire 5 of less than 0.7 mm² which is woven into a patternwith the wire legs 12.5 mm apart as shown in FIG. 11, and thensandwiched between two sheets 6 of polythene. Looped ends 7 of the wire5 are used as terminal connections. This system will operate atapproximately 4.5 volts and 3 amps. The two sheets of polythene areglued or fused together to hold the wires in position. This 15 watt unitis designed for bottom heating for home propagation of seedlings or potplants, pet box warmers or foot warmers for those who own a batterycharger. This variation is unique in that it is specific to a homebattery charger and incorporates the close spaced small diameter wiringto provide even heat, low voltage, steel wire and polythene cladding ofthe invention. The layout of wire shown in FIG. 11 and polythenecladding allows the heating pad to be rolled up into a parcel (tube)that incurs low cost postage fees.

In FIGS. 12, 13 and 14, a mechanical method of construction for theelement of FIG. 11 is proposed which takes two rolls of polythene 6pre-stamped with instructions and advertising and feeds them each sideof the loom frame 9 on which the heating element is threaded. Seal barrollers 10 then run over the frame, sealing the straight lengths of wirebetween the polythene sheets 6. Edging machines 11 then clad the wireloops each side and the heating panels are then cut and packaged readyfor dispatch. The process is illustrated diagrammatically in FIGS. 12and 13. The loom frame shown in FIG. 13 moves forward between the sealbar operation area then after sealing releases the wire and withdraws tobe re-threaded. An alternative to using seal bar rollers is to use apair of plates which seal each panel in a single action. The advancedweb can be cut using a cutting device 12.

It can be noted from FIG. 14 that the polythene sheets protrude beyondthe seal bars. After the main sealing is done the loom frame releasesthe wires and the product moves forward to have the edges sealed.

Aspects of the present invention have been described by way of exampleonly and it should be appreciated that modifications and additions maybe made thereto without departing from the scope thereof as defined inthe appended claims.

I claim:
 1. A method of heating a floor, wall or ceiling by means of anelongate wire mesh or netting heating element including a plurality ofphysically and electrically connected longitudinal and crosswise wires,said longitudinal and crosswise wires being welded together to form amesh heating element of at least 10 meters length and at least 75millimeters width and with a thickness of less than 2 millimeters, saidmethod of heating comprising the steps of:(a) fabricating a mat ofwelded mesh heating element comprising a plurality of longitudinal wiresconnected together through welding to a plurality of cross wires, (b)spreading said element evenly across an area to be heated through whichan electrical current is driven, and (c) connecting the two ends of theelement to a power source with a voltage of less than 40 volts.
 2. Amethod as claimed in claim 1, wherein the heating element is uninsulatedand encased in a settable medium.
 3. A method of heating as claimed inclaim 2, wherein the settable medium is grout.
 4. A method of heating asclaimed in claim 2, wherein the settable medium is a bedding compound.5. A method of heating as claimed in claim 2, wherein the settablemedium is an adhesive.
 6. The method as claimed in claim 1, wherein theheating element is spread over a surface area to be heated, the surfacecomprising a wall, floor or ceiling.
 7. The method as claimed in claim1, including the steps of laying the heating element over a floor, andlaying a floor covering sheet over the heating element.
 8. A method asclaimed in claim 1, wherein the heating element provides 2500 cm² to6000 cm² of surface per 100 watt output for every square meter to beheated.
 9. A method of heating as claimed in claim 1, wherein the powersupply is capable of converting mains supply voltage to less than 40volts.
 10. A method as claimed in claim 1, wherein connectors are usedto connect the element to the power source, and the connectors are lessthan 2 millimeters thick.
 11. A method as claimed in claim 1, whereinthe cross-wires increase the heat dissipating surface area of theheating element.
 12. A heating element, comprising two sheets ofmaterial; a wire netting or mesh heating element sandwiched between saidtwo sheets of material; the wire netting or mesh having terminal endsadapted for connection to a power source having a voltage less than 40volts; and said wire netting or mesh including a plurality of physicallyand electrically connected longitudinal and crosswise wires, said wiresbeing welded together to form a mesh heating element of at least 10meters length and at least 75 millimeters width and with a thickness ofless than 2 millimeters.
 13. A method of heating a surface, comprisingthe steps of sandwiching a small diameter mild steel wire heatingelement formed from a mesh of longitudinal and crosswise wires weldedtogether between two sheets of material, the heating element having alength of at least 10 meters and a width of at least 75 millimeters anda thickness of less than 2 millimeters, said heating element beingpositioned evenly between two sheets under the area to be heated, anddriving an electrical current through the heating element by connectingthe two ends of the element to the terminals of a transformer convertingmeans capable of converting mains voltage to between 6 to 24 volts, andwhich has less than 150 VA capacity wherein the diameter of said wirebeing approximately 0.7 millimeters.